This invention relates to a radio channel control method in a cellular system for providing a plurality of kinds of communication services, such as a telephone service and an Internet connection service.
In recent years, a cellular system provides a data communication service such as an Internet connection service in addition to a communication service such as a telephone service and a TV telephone service. In telephone communication as speech communication, information is generally transferred by circuit switching (CS). In data communication, information is generally transferred by packet switching (PS). In the following description, a call in the speech communication and a call in the data communication will be referred to as a CS call and a PS call, respectively.
In the cellular system of the type, it is desired to minimize call loss of CS calls and to accommodate as large traffic of PS calls as possible. In either of the CS call and the PS call, it is desired that a failure rate of handover is smaller than that of call connection. This is because, in case of the CS calls a failure in handover results in interruption of telephone conversation so that a user can not accomplish an object of communication. In case of the PS call, a failure in handover results in interruption of transmission of data and often requires re-transmission of the data. This means that a resource used in first data transmission is useless. Further, it is desired to maintain a communication quality of a channel currently used.
In order to satisfy the above-mentioned demands, a typical cellular system adopts a control method as shown in FIG. 1. Specifically, a load of each cell is calculated and compared with a predetermined call setup threshold value. If the load exceeds the call setup threshold value, no channel is assigned. If the load is smaller than the call setup threshold value, a channel is assigned. Herein, a PS call setup threshold value is lower than a CS call setup threshold value. If the load is greater than the PS call setup threshold value, a new channel is not assigned to PS calls so as to reserve resources for accommodating CS calls. Thus, a call loss rate of the CS calls is reduced. Further, it is assumed that a mobile station moves from a cell where a call is originated and a channel is first assigned to the call to another cell as a destination cell and that the call is handed over. In this event, a new channel is assigned if a load of the destination cell is lower than a handover threshold value. The handover threshold value is selected to be higher than both of the CS call and the PS call setup threshold values so that the probability of failure in handover is reduced.
However, upon carrying out handover of the CS call or the PS call, a channel may be assigned when the load of the cell is higher than the CS call setup threshold value. Thus, the load of the cell exceeds the CS call setup threshold value at a high probability. This results in an increase in call loss rate of CS calls.
In recent years, in realtime speech communication also, information may sometimes be transferred by packet switching. In case where the CS call and the PS call are replaced by a call in a realtime communication service of a conversational or a streaming traffic class and a call in a non-realtime communication service of an interactive or a background traffic class, respectively, there is a similar drawback to that described above. Further, in case where the CS call and the PS call are replaced by a call in a high-priority communication service and a call in a low-priority communication service, respectively, there is a quite similar drawback.
Japanese Unexamined Patent Application Publication (JP-A) No. 2002-217986 (Patent Document 1) discloses a method of changing circuit switching and packet switching. Specifically, when a new call is originated, the call is at first assigned with a packet-switched communication channel. If a communication time of the call using the packet-switched communication channel exceeds a predetermined time or if the frequency of occurrence of packet collision in the call exceeds a predetermined value, the packet-switched communication channel assigned to the call is changed to a circuit-switched communication channel.
Japanese Unexamined Patent Application Publication (JP-A) No. H11-18175 (Patent Document 2) discloses a communication method in which a communication rate is automatically controlled. Specifically, in case where data transmission is cyclically carried out between a master station and a plurality of slave stations according to a CDT (Cyclic Digital Telemeter) system, the master station measures a processing load and a communication rate of each channel is changed depending upon the processing load.
Japanese Unexamined Patent Application Publication (JP-A) No. 2002-503068 (Patent Document 3) discloses that a channel rate assigned to a call connection request is determined by an occupied cell capacity.